Methods of forming charges of solid fuels by hydraulic compression

ABSTRACT

METHOD OF FORMING A SOLID PROPELLANT CHARGE WHEREIN FLUID PRESSURE IS APPLIED AGAINST A CASING THAT CONTAINS THE PROPELLANT INGREDIENTS TO OBTAIN UNIQUE DENSITY GRADIENTS IN THE CHARGE.

Oct. 2, 1973 A. c. LOEDDING 3,753,291

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INVENTOR. ALFRED C. LOEDDING ATTORNEY United States Patent fice 3,763,291 Patented Oct. 2, 1973 3,763,291 METHODS OF FORMING CHARGES OF SOLID FUELS BY HYDRAULIC COMPRESSION Alfred C. Loedding, Dayton, Ohio, assignor to Twin Fair, Inc. Filed Mar. 10, 1951, Ser. No. 214,921 Int. Cl. C06b 21/02 US. Cl. 264-3 R 8 Claims ABSTRACT OF THE DISCLOSURE 'Method of forming a solid propellant charge wherein fluid pressure is applied against a casing that contains the propellant ingredients to obtain unique density gradients in the charge.

This invention relates to improvements in methods of forming charges of solid fuels, and is directed more particularly to the forming of charges for use in rocket or reaction types of motors or the like. I do not intend however to limit such charges entirely to those which are self-suflicient in oxygen content to be able to burn when excluded from the atmosphere, as I can, by practising the hereinafter disclosed method, produce charges of solid fuels which require at least a portion of atmospheric oxygen, as in the case of an augmentor or ejector, and further as a prime fuel for ramjet power plants for the supersonic flight of missiles, aircraft, etc.

One object of the invention is a method of forming solid fuels into uniform elongated bodies wherein the density of the material is greatest at the outer surface thereof and decreases toward the center thereof.

Another object of the invention is the formation of a mass of solid fuel into a predetermined shape, applying a flexible coating over the outer surface thereof, and subjecting said surface to fluid pressusre in order to compact said mass gradiently.

A further object of the invention is the formation of a charge of solid fuel in the form of a solid cylinder with convex ends, the formation of a conical depression in one of said ends, and the mounting of an iginition cone therein.

Another object of the invention is the preformation of a charge of solid fuel by tamping a required quantity of the materials into a mold, slowly ejecting the molding, for example by moving a piston upwardly in the mold, spraying a flexible coating onto said molding, and subsequently submerging the latter in a fluid medium in a pressure vessel, and imposing hydraulic pressure on said fluid medium.

Another object of the invention is the formation of a charge of solid fuel by tamping the required quantity of the fuel, which may be granular in form, into a scalable flexible envelope which has been positioned in an upright mold, sealing said envelope, removing the same from said mole and subjecting it to hydraulic pressure to give it an inverse density gradient.

Other objects and advantages of the invention will be apparent to those skilled in the art upon a study of this specification and the accompanying drawings.

Referring to the drawings:

FIG. 1 is a curve plotted between rate of burning, and chamber pressure (p.s.i.);

FIG. 2 is a curve plotted between linear rate of burning in inches per second, and p.s.i. of chamber pressure;

FIG. 3 is a diagrammatic representation of a charge, illustrating the manner in which burning is effected;

FIG. 4 is a cross-sectional view taken along the line 4-4 of FIG. 3;

FIG. 5 is a view of a formed charge made according to the invention and showing an ignition cone formed in one convex end thereof;

FIG. 6 is a fragmentary view showing how different initial thrust values are obtained;

FIG. 7 is a curve plotted between the thrust in pounds, and time in seconds;

FIG. 8 is a view of a charge made according to the invention and showing the propagation of the burning surface initiated by cone ignition;

FIG. 9 is a curve plotted between the diameter of the charge and the required forming (pressing) pressure;

FIG. 10 is a cross-sectional elevation of a cylindrical mold into which the material is tamped to form a charge, said mold having a piston therein for forming one convex end and at the same time forming the conical depression to accommodate the ignition cone;

FIG. 11 is a cross-sectional elevation of the mold of FIG. 10 and showing an annular spraying device for coating the molding as it is pressed out of the mold by means of the piston;

FIG. 12 is a view similar to FIG. 11 except that the molding has been ejected about half way out of the mold; and

FIG. 13 is a cross-sectional elevation of a pressure vessel into which the prepared charge is positioned and subjected to hydraulic pressures for creating within the body the charge an inverse density gradient.

In connection with the application of solid fuels to rocket or reaction-type motors in general, it will be understood that this invention covers either fuels which are self-sufiicient in oxygen content, which can burn when excluded from the atmosphere, or fuels that may require a portion of the atmospheric oxygen to be supplied to support combustion as in the case of an augmentor or ejector, and further as a prime fuel for ramjet power plants for supersonic flight of missiles and/or aircraft.

PROPELLANT The propellant may consist of any number of ingredients that can be blended and formed into a suit able hard mass by the application of pressure without the need of bonding agents such as cements and plasticizers. Typical of such ingredients are those used in the wellknown black powder, i.c., sulfur, charcoal, and potassium nitrate (SCKNO Like black powder the preferred ingredients are those that will not be sensitive to shock. That is, chemical dinoculation or combustion will not occur as the result of shock or of a detonation wave, regardless of the magnitude thereof. Further, the propellant when formed into an desired configuration will always burn progressively, particularly under high pressure that would be characteristic of black powder, as may be clearly seen in FIG. 1 of the drawings.

FABRICATION OF THE PROPELLANT CHARGE (A) Die method One conventional method of shaping propellant charges into various configurations is the use of a ram or plunger that is forced into a cylindrically shaped die that contains the loose or unpacked propellant ingredients or powder grains. This can also be accomplished b the extrusion process, i.e., forcing the propellant through a suitable die when in a somewhat plastic state. To date, however, none of these methods has proven satisfactory, particularly for relatively large propellant charges for rocket motors that would be suitable for JATO application.

The ram-die method has proven unsatisfactory for the following reasons:

(1) Inverse density gradient for all cross-sectional elements along the cylindrical axis. That is, the density or compactness of the powder is relatively lower towards the outside surface (center being hardest). This adverse condition is the result of wall friction that retards the flow of the powder along the walls of the die. Malfunction of the rocket motor will usually occur as a result of the irregular burning area parallel to the intended burning axis; according to the laws of combustion, the burning rate is proportional to the exposed area and hence the density. That is, combustion will proceed at a greater rate along the outside of the propellant charge where the density is lower (greater porosity) which will result in excessive pressure and malfunction. If shock-sensitive powder is used, the entire motor will burst like an explosive bomb and cause considerable damage.

(2) Danger of inadvertent combustion due to heat generated during the pressing operation and also sparking as a result of die defects or impurities in the powder. Also static charges of electricity can be set off during the pressing operation.

(3) High cost of dies, rams and associated equipment because of:

(a) Strong and large sizes required due to great pressures (10,000 to 20,000 p.s.i.).

(b) Handling difliculties requiring special equipment and skilled operators.

Precise machining operation to achieve smooth surfaces and relief through tapering of the dies to facilitate ejection or removal of the formed charge.

(d) Wastage through formation of cracks due to difficulty to control pressure release during ejection of the charge from the die or mold.

(e) Need for large presses that are practically prohibitive for large motors.

(4) Poor production rate because of:

(a) The mechanics of loading the die and care required for proper and safe handling.

(b) As yet, the unachieved ability to press relatively large charges by a single operation, thus requiring a number of segments or built up charges to form a single unit. No satisfactory method has been evolved to accomplish this either by mechanical means or bonding agents.

(B) Extrusion method This method is similar to Wire drawing. That is, the somewhat plastic propellant must be forced through a die. If the plasticity is inadequate, numerous cracks will form causing uncontrolled burning. To date, no propellant possessing the desired degree of plasticity for extrusion has proven satisfactory for JATO application. The proper degree of plasticity is difficult to control after curing and during storage so that irregular burning and hence malfunction will likely occur. Further, to date all propellants of a plastic nature are shock sensitive to a more or less degree rendering them too hazardous for large JATO units and for guided missile propulsion. Numerous fires have resulted in attempts to extrude various known propellants, due to friction and/or static electricity discharges.

(C) Cast method This method is most popular to date, but is not proving satisfactory because:

(a) Limited size due to heat generated during the cooling and setting stages.

(b) Texture of the cross-sectional segments that result in a convex cone shaped burning surface which has a tendency to increase and offer greater burning area, and therefore a rise in pressure and thrust.

(0) Danger during melting and blending of the ingredients.

(d) Poor temperature range inherent in any ingredient that can be melted and cast to +120 F.).

(e) To date, all ingredients that can be melted and cast require powerful oxidizing agents such as potassium perchlorate (KCLO which renders the charge shock sensitive (combustion can be initiated by shock alone).

(f) Excessive cost and need for critical ingredients to meet proper casting and combustion properties.

(g) Storage precautions to assure that the charge is stored vertically to prevent cold rflow inherent in cast propellants. Thus irregular or improper exposed areas are presented causing excessive pressure and malfunction.

(h) To date, all cast propellants have proven diflicult to ignite and therefore require special ignition units.

Referring to FIGS. 10 to 13 inclusive, which illustrate my new principle, a cylinder 15 has an interior bore 16 in which a plunger member 17 forms a working fit. The piston 17 has a piston rod 18 which extends downwardly into a cylinder 19 and carries on its lower end a piston 20.

The piston 17 is preferably formed so as to define a rounded or convex nose 21 on the molding. Extending into the rounded or convex nose 21 is a depression 22 for containing the ignition charge, termed herein the ignition cone.

As will be pointed out hereinafter, I may form the ignition cone in accordance with the thrust desired. For example, if the initial thrust is to be low, the depth of the ignition cone is shallow; if the initial thrust is to be medium, the ignition cone is of a medium depth, and if the initial thrust is to be high, the ignition cone is of substantially greater depth. There are other effects due to the ignition which will hereinafter be explained.

Returning to FIG. 10, the granular material 23 is placed in the cylinder 15, with the piston 17 positioned at the lower end thereof and the proper quantity of material is tamped into the cylinder.

After the proper amount has been so placed in the cylinder, the upper end of the material may be rounded by hand or with a plastic form so that it will present a convex surface upwardly. Following this, the piston 20 in the lower cylinder 19 is forced upwardly and the tamped material is gradually pushed upwardly from the cylinder 15. As it is moved upwardly, a suitable annular spray device 24 applies a coating to the entire external surface of the molding.

As an alternative the mold, that is to say the cylinder 15, may have placed within it a latex or other flexible bag, and the materials may be tamped into this bag instead of in the cylinder alone, and when sealed, the bag with its contents may be ejected.

The charge of material with its coating, regardless of whether it is a bag per se or a sprayed external coating thereon, is placed into a pressure vessel such as the one shown in FIG. 13. This pressure vessel has a flanged cylindrical body 26 to which is attached a suitable head piece 27 by means of suitable cap screws 2.8 with a sealing gasket 29 between the flanges to prevent leakage. The pressure vessel contains a fluid medium 30 into which the molding of the charge is submerged. Operatively connected to the pressure vessel 26 is a suitable hydraulic pump shown diagrammatically at 31. The charge as initially placed in the pressure vessel has a size end outline which is approximately that shown in dotted lines in FIG. 13, and after the charge has been subjected to the hydraulic treatment it assumes the general size and outline shown in solid lines in FIG. 13.

By means of the above described principle, the following advantages and results are obtained according to my invention:

(A) Principle involved This method employs the established hydraulic principle that fluid pressures are distributed equally and in all directions. This pressure is brought to bear against a casing that contains the propellant ingredients, thus producing a highly desired density gradient. That is, since the pressure is exerted from the outside towards the center, the hardness or density will be greater on the outside than at the center because of the granular or non-plastic nature of the propellant ingredients.

(B) Aifords use of many types of ingredients In other words, the ingredients can be selected that are crystalline and granular like black powder. Therefore, a great many different kinds of ingredients become available for JATO use since this method can consolidate mechanical mixes or blends of two or more ingredients, one of which is the oxidizing agent which may be the non-shock sensitive type such as potassium nitrate or sodium nitrate. The formula for black powder is approximately 75% of KNO (or NaNO approximately willow charcoal, approximately 10% sulphur.

Another explosive suitable for solid fuel is approximately 70% (KNO or NaNO and approximately 30% charcoal.

Another explosive suitable for solid fuel is dry Bullseye granules (Hercules).

Still another explosive powder suitable for solid fuel is nitrocellulose powder (ground under water).

Yet another explosive which is sometimes called semismokeless powder is produced by King Powder Company, of Cincinnati, Ohio, which is comprised of 10 to nitrocellulose which is sold under the name of Pyro and the balance is comprised of black powder. Any of the above mentioned explosive powders may be formed into solid fuel for rockets by the method herein disclosed. Bonding agents may be eliminated and thereby avoid dilution of the energy content or introduction of other undesirable qualities that could cause poor storage characteristics or limit the temperature range.

(C) Affords desirable temperature range Black powder propellant charges have been successfully fired in rocket motor chambers at 100 F. and up to +170 F. This temperature range can easily be duplicated with other ingredients, i.e., substituting specially refined powdered coal for the willow tree charcoal and synthetic waxes for the sulfur and using either KNO or NaNO as the oxidizing agent.

(D) Affords lower pressing pressures This method will further permit lower forming pressures and hence reduce the possibility of heat shock. That is, the charge will not have a tendency to disintegrate as a result of the sudden application of heat such as during the initial stages of ignition. The pressing pressures are expected to be reduced from 15,000 or 20,000 p.s.i. to as low as 8,000 psi. This value may be further reduced to approximately 5,000 p.s.i. by blending large portions of guanadine nitrate (H NC(NH)NH HNO with the solid fuel.

(E) Other outstanding advantages of my method (a) One important advantage that is obtained as a result of this method, which is not achieved by others is the variation in burning rate without changing the composition of the ingredients. This is achieved simply by the using of less pressing pressure and allowing the linear rate of buring to increase to within safe limits that can be determined best by actual experiments.

Since the thrust of a rocket motor is directly proportional to the mass flow of gases per second, times the exhaust velocity in feet per se,

T=M V (lbs.)

Greater thrust is achieved for a given or set exhaust velocity (limited by B.t.u. or caloric heat content of the propellants) by increasing the linear rate of burning. This is only permissible if it can be controlled. This is possible by reason of this proper density gradient, i.e., greater density increasing toward the outside surface of the charge.

(F) Elastic casing becomes integral part of propellant charge One of my preferred methods of priming the charge is to first place the blended ingredients in lightly constructed molds such as those shown in FIGS. 10 to 12 described above (plastic or hard wood), and press slightly by use of a ram or centrifugal force. This pressure may be as low as 25 p.s.i. to psi. depending upon the ingredients and the size of the charge. The slightly packed charge is now ready to be pushed slowly out of the mold by any suitable means so that an elastic coating can be sprayed, painted, or dipped to produce a thin coating sufiicient to protect the powder from the hydraulic fluid or the pressing medium. This coating will also serve to retain the desired shape until it can be pressed to the required density. As the ingredients are pressed it is clear that the elastic coating will become imbedded or mechanically bonded to the outside surface of the charge to assure controlled parallel turning (like a cigarette).

Since the diameter is reduced approximately 50% from the slightly packed condition, it is also clear that the thin (approx. ,4 coating will become thicker and present a practical covering to:

(1) Form an excellent seal to prevent the charge from absorbing atmospheric moisture and thereby permit longtime storage.

(2) Provide an insulated coating on the inside of the combustion chamber to protect it from the extreme temperature.

(3) Allow the charge to fit snugly in the combustion chamber and then protect it against shock-and also prevent hot gases from leaking along the sides of the charge.

(4) Permit simple low-cost packing and shipping containers.

(5) Protect the charge from becoming chipped, scratched or otherwise damaged.

(6) Tend to cool the nozzle by breaking down into a fluid that will be entrained and made to flow along the inside surfaces of the nozzle. This can be achieved by proper selection of the composition of the elastic coating.

(G) Simplified ignition Generally, mechanically mixed powders, particularly black powder, are well known for their ease of ignition. One end of the charge can be recessed in the center to form a conically shaped hole. This hole can be formed automatically during the pressing operation by inserting a suitably shaped plug or mandrel that can be removed and then replaced by a simple electrical resistance wire arrangement, when either ready for use or immediately after the charge is formed. Experience and requirements of the using agency will best determine the method to be followed. The lead-in wire can be allowed to extend through the nozzle or may be attached to fittings on a diaphragm across the outer opening of the nozzle.

(H) Controlled initial thrust The initial thrust magnitude may be controlled to give certain characteristics such as a very low, medium or high thrust over a range of approximately 1 or 2 seconds. Low, medium and high initial thrust values are obtained by the depth of the ignition cone. This cone will assure controlled turning that will propagate readily into the desired pattern of parallel segments. (See FIGS. 6 and 8.) Actual dimension of the cone will be determined by experiment for each given blend of propellant ingredients and ambient temperature conditions.

(I) Eliminates large and costly presses This method requires no press, as in the case of the die and ram arrangement, which represents a great savmg, safety and simplified production.

(J) Production possibilities A number of pressure vessels can be connected to a single hydraulic pump unit so that a multitude of charges can be formed simultaneously.

(K) Size not limited Sizes as big as feet in diameter and feet long are visualized for guided missile use.

(L) Pressure vessels to form charges are practical Since the pressure vessels do not have to be moved about, they can be constructed of reinforced concrete with metallic liners and located underground if desired.

Preforming can be resorted to in order to reduce the size and permit smaller pressure vessels.

(M) My method has many uses and advantages in addition to the above Other items may be pressed into various configurations by use of a rubber sheathing in place of the elastic cement liquid latex, and various types of synthetic rubber cements.

(a) Graphite nozzles for rocket motors.

(b) Smoke pots for signalling, etc.

(c) Most any item composed of mechanically mixed ingredients that are now being formed by usual methods of extrusion or die-ram arrangement.

Higher rate of turning possible through this method means relatively smaller diameter combustion chamber for a given thrust. Since stress, and hence thickness, of chamber wall is directly proportional to the diameter, a lighter motor will be possible. A further gain can be achieved by reduced burning pressure from approximately 1,500 p.s.i. to 500 p.s.i.

In addition to the above improvements, I propose to utilize basic asphalt which is a high molecular polymer comprised of carbon and hydrogen. However, this material softens above 110 F. and due to its lack of rigidity, it does not burn on a fixed ratio. At very cold temperatures it cracks causing uneven burning and under the best conditions, it produces smoke.

As a result of work in this direction, a new type of fuel which can be poured cold or ejected through dies or nozzles, will harden into a material which will not soften below 222 F. or crack below l50 F. and will have a high B.t.u. value with uniform burning and with no smoke. This fuel can be used alone or in combination with asphalt, using the same percentage of KNO S and no plasticizer is required. Of course, asphalt has an advantage in the price per pound, and asphalt can have added to it a non-flowing material such as rubber from 10% to 40% by volume. This rubber may be of CTRS or natural crude type. Either one can be used and will overcome cold, cracking, heat, and the slumping off of asphalt.

The new type of fuel is a methyl-methacrylate mixture of monomer and polymer. The advantage of this fuel is that less KNO is required for the oxidation action, and the fuel will burn without smoke.

Although I have herein shown and described by way of example several methods of forming charges of solid fuel, it will be obvious that many changes may be made in the arrangements, the combinations thereof and the proportions of the materials within the scope of the following claims:

What is claimed is:

1. The method of forming solid fuel bodies which includes the step of preforming said solid fuel into a shape generally resembling the final shape, the step of applying a conforming elastic coating to the external surface of said body, and the final step of subjecting said coated body to a fluid pressure between 5,000 and 8,000 p.s.i.

8 acting upon every portion of its external area thereby producing a density gradient in said body, the density being greatest adjacent to said external surface.

2. The method 'of producing a charge of solid fuel which includes the step of preforming a body from an explosive mixture in the form of a cylinder with convex ends and cavity means in one end of said body, the step of subjecting said body to the influence of a fluid under hydraulic pressure in the neighborhood of 5,000 p.s.i. and thereby producing in said body a density gradient from the outside inwardly, by subjecting said body to the influence of a fluid under hydraulic pressure of not less than 750 p.s.i., the step of placing ignition means in said cavity so that flame propagation in said body is encouraged along an axially advancing concave burning surface.

3. The method of producing a charge of solid fuel from an explosive powder which comprises the step of producing a charge of solid fuel from an explosive compound which comprises the step of preforming said compound into a body having convex ends, one end of which has a cavity formed therein, the surface of which initiates a concave burning pattern in said body, the step of submerging said body in a fluid medium under hydraulic pressure of between 5,000 and 10,00 p.s.i. and thereby producing an inherent density gradient in said body from the outside toward the axis thereof and the final step of applying ignition means in said cavity to initiate said burning.

4. The method of forming a charge of solid fuel for a propellant which comprises the step of initially forming said charge into an elongated body of a predetermined size, the step of applying a conforming flexible coating on the outer surface of said charge to form a flexible skin on said body, the step of submerging said body on a fluid medium, which is to burn along with said charge and promote parallel burning thereof, and the final step of subjecting said body to hydraulic pressures greater than 500 p.s.i. to cause at least the outer surface of said body to become uniformly dense pressures of between 5,000 and 15,00 p.s.i.

5. The method of forming a charge of solid fuel, from an explosive compound, which comprises initiall forming said charge into a body of a predetermined size, the step of applying a conforming flexible coating on the outer surface of said charge to form a flexible skin, the step of submerging said body in a fluid medium in a confined space and the final step of subjecting said space and said fluid medium to hydraulic pressures in the neighborhood of 5,000 p.s.i. to render at least the outer surface of said body uniformly dense, said skin also functioning to encourage parallel burning of said charge.

6. The method of forming a charge of solid fuel from explosive powder which includes several steps, one of which comprises the subjecting of a pre-formed body to a hydraulic pressure of between 5,000 and 8,000 p.s.i., whereby every portion of the area of said body is in free contact with said pressure and a hardness is produced in the body of said charge.

7. The method according to claim 6 in which the step producing the hardness, comprises submerging the charge in a fluid medium within a pressure vessel and the step of impressing the aforesaid hydraulic pressure of between 5,000 and 8,000 p.s.i., upon said vessel and its contents for a predetermined time period.

8. The method of forming a charge of solid fuel which includes an initial step of forming an explosive powder into an elongated body with convex ends, enclosing said body into hermetic flexible envelope, and the step of hydraulically reducing the mass of said body in all directions by subjecting it to fluid pressure in an enclosure with pressure imposing means exerting predetermined pressures of between 5,000 and 15,000 p.s.i. thereon.

(References on following page) References Cited 10 OTHER REFERENCES Bebie: Manual of Explosives, Military Pyrotechnics and Chemical Warfare Agents, Macmillan Co., NY. (1943), pp. 33-35.

Healy: The Black Powder Rocket Charge, Astronautics, No. 53, October 1942, pp. 3-7.

Pendray: Introduction to Jet Propulsion, Journal of the American Rocket Society, No. 64, December 1945, pp. 15-16.

CARL D. QUARFORTH, Primary Examiner S. HELLMAN, Assistant Examiner US. Cl. X.R. 

